Cosmetic cleanups in the BSON writer (stale comment, inconsistent signature)

[banaboi]

Two non-functional cleanups in include/nlohmann/detail/output/binary_writer.hpp:

1. Remove stale // boolean comment in write_bson_entry_header. That function
   writes the header for every BSON element type, not just booleans. The comment is
   left over from when the code was boolean-specific.

2. Change write_bson_unsigned to take std::uint64_t instead of const BasicJsonType&.
   It reached into j.m_data.m_value.number_unsigned three times, while every sibling
   (write_bson_boolean, write_bson_double, write_bson_integer, write_bson_string)
   takes the plain value. The BasicJsonType parameter was only needed as diagnostic
   context for the out_of_range.407 throw that Making Code cleaner #4286 removed, so it no longer earns
   its keep. The call site now reads like its neighbours.

While there, fixed the doc comment on the adjacent calc_bson_unsigned_size, which still
referred to "the unsigned integer in @a j" despite already taking a value parameter.

No behaviour change.

Closes #5312

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• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
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[nlohmann]

Looks good to me.